The invention relates to a plotter having a flat bed and having a tool carriage which can be moved in a first co-ordinate direction and having a tool holder which can be moved on the carriage in a second co-ordinate direction which is orthogonal to the first co-ordinate direction.
Flat-bed plotters of this type are used for drawing, cutting, milling or similar operations, with the appropriate tools being set in the tool holder. The flat-web material to be processed, e.g. paper, sheets, cardboard or textile webs, are situated on the flat bed. The movements of the tool holder in the first and second directions are carried out and controlled by a program. Drive motors are provided to drive the tool carriage in the X direction and the tool holder in the Y direction, said drive motors being controlled via a control circuit in accordance with the predetermined program.
The conversion of the rotary movement of the drive motors into the linear movement of the tool holder is accomplished in different ways on existing plotters. A first type of plotter, for example, employs toothed racks for the movement of the tool carriage in the X direction, said racks extending at the edge of the flat-bed, outside the working surface. These racks are considered to be robust components. However, they are expensive to manufacture, thereby increasing the price of the apparatus considerably. Toothed belts are also customary. However, they cannot guarantee the desired rigidity and freedom from play of the drive.
The second customary type of drive connection between the co-ordinate drives and the tool holder comprises cable strands at both ends of the tool carriage. The cables, in general steel cables, like the racks, extend on both sides at the edge of the flat bed. They are each passed via a driving pulley and via one or more deflection pulleys and engage on the tool carriage in both directions along the X axis in an endless loop. Although single-core wires deliver the better values for this purpose as regards tensile strength, stranded wires are predominantly used since they have the better flexural elasticity. The cables can thus be passed via deflection rollers of smaller diameter, a more compact overall construction thereby being achieved.
However, the use of stranded wires leads to the problem of plotter resolution and accuracy. Plotters of this kind should have a high addressable resolution, e.g. of 0.005 mm, and have a high static repeating accuracy when being positioned, e.g. of about 0.01 mm. Such values make very high demands on the stiffness, i.e. the spring constant of the cables. For a given mass of the carriage system and for a given acceleration, the stiffness is responsible for the dynamic accuracy, i.e. the line quality of the plotter. As an example, the acceleration can quite easily reach 10 m/s.sup.2, while the speed of travel is, for example, 1.0 m/s.
In known large-format plotters, only simple cable drives having steel cables having a nominal diameter of about 2 mm have been employed for driving the tool carriage. In this manner, only relatively modest values for the rigidity of the carriage system were achieved, with the result that this kind of drive could only be used for plotters of an inferior accuracy.